1. Field of the invention
The present invention relates to a light-tight cassette or magazine (hereinafter called "cassette") for holding and dispensing a roll of light-sensitive photographic material.
The invention is particularly suitable for use with phototype-setting apparatus.
2. Description of the prior art
Cassettes in the form of a generally rectangular box having a peripheral light-tight slot via which light-sensitive material can be drawn from a roll located in the box are known. The light-tightness of the dispensing slot is important for preventing light from entering the cassette and fogging outer convolutions of the roll of light-sensitive material during the removal of the cassette from its wrapper and during day-light loading or unloading of the cassette into or from a photographic exposure apparatus.
Most present-day cassettes are made from plastics or cardboard. The peripheral wall or shell of the cassette has in-turned plastic or cardboard lips forming the dispensing slot, and light-tight sealing of this slot is commonly effected by elastically deformable linings, more particularly, strips of velvet. These strips are adhered to the lips by means of glue, in particular a pressure-sensitive, also called self-adhesive glue, provided at the backside of the light-sealing strips.
The described glue can sometimes cause difficulties, in particular at the first pulling out of a length of a fresh roll of photographic film or paper. As a matter of fact, it may happen that the film comes into contact with the longitudinal edge of a light-sealing strip at the inside of the cassette and thereby touches and adheres to the corresponding edge of adhesive that is exposed at such place. The exposed edge of adhesive may have been caused by an excessive pressure during the application of the light-sealing strip to the lip, by a layer of adhesive that is too thick, by creeping behaviour of the adhesive, etc.
Pulling-out a film which has become adhered too the adhesive becomes difficult, and increased effort by the operator, in order to obtain a length of film sufficient for the proper insertion of the film in the nip of the transport rollers of an exposure apparatus, aggravates the situation still more and finally may cause the film to bend back a marginal portion of the light sealing strip from the lip whereby this strip portion enters the dispensing slot and blocks any further movement of the film.
It is believed there are two important causes for the described defect.
First, the film as it is drawn from the roll and enters the dispensing slot of the cassette, follows different angular directions depending on the extent to which the roll has been used. The limits of the range of different directions are formed by the film paths that are tangent to a fresh or a full roll at one extreme, and to an empty roll at the other. The dispenser slot can accept a wide range of paths since the lips of the slot usually will diverge or flare inwardly, i.e. in the direction towards the roll, but it will be understood that for the extreme film directions as described, the angular clearance between a light sealing strip and the oncoming film is small, and thus minor deviations of the film from its straight path may suffice for bringing the film in contact with the radially inwardly edge of the light-sealing strip and then with the edge of the adhesive backing layer itself. A well-known cause of film deviation from its straight path from the roll towards the dispensing slot is partial unwinding of the roll of film under the influence of vibrations that may occur during the transport of the cassette.
Second, there are known cassettes in which the premature unwinding of the film roll during the transport is prevented by a locking member or key that locks the rotation of the core prior to the first use of the cassette. In one form, such locking member is ruptured by the operator when pulling out a certain length of film from the cassette, which causes overtensioning of the member and eventually its rupturing. It has been shown that when the key finally breaks and the film resistance drops, the first pull by the operator is often too strong so that an important slack is produced in the film end between the roll and the dispensing slot which may cause the film to touch the edge of one or both light-sealing strips, and thereby also an edge of the adhesive layer.
It is possible to increase the curvature or divergence of the inner ends of the lips of the dispensing slot in order to widen the entrance opening for the film, but this is not desirable in practice since it complicates the correct application and glueing of a light-sealing strip to the lips.